Our International application PCT/AU99/01028 discloses a system in which the location of a disturbance to a fibre and, in particular to a communication fibre can be determined. This system utilising counter-propagating optical signals so that light signals propagate in both directions along a waveguide. Any attempt to disturb the waveguide will cause a change in the counter-propagating signals and that change can be detected by detectors so that a time difference between receipt of the modified counter-propagating optical signals enables the location of the disturbance to be determined.
Our International application number PCT/AU00/00382 discloses a system in which both a sensing signal and a communication signal can be launched into a single waveguide and transmitted along the waveguide with minimal losses to both the sensing signal and the communication signal.
Our International application number PCT/AU00/01332 discloses a method and system in which perimeter barrier elements such as fence sections are spring mounted for limited movement. An optical fibre is connected to the elements so that any attempt to break in or tamper with the fence causes the element to move against the bias of the spring which in turn moves the optic fibre so that a change in a parameter of the light travelling through the fibre can be detected in order to provide an indication of the intrusion or tampering.
The contents of the above four International applications are incorporated into this application by this reference.
The perimeter barrier technique disclosed in the abovementioned International application provides an extremely efficient monitoring system and method for perimeter barriers which include fences or other physical elements which are intended to provide a barrier against ingress of individuals. Since the above invention operates by spring mounted fence elements having a fibre in proximity to the fence element so that movement of the element moves the fibre, it is necessary that, in the earlier invention, the perimeter barrier be formed by a physical structure to which the fibre is attached. The present invention relates to a perimeter barrier system in which there is no physical barrier required in order to operate the detecting system and which is therefore suitable for location in the ground to provide security to a perimeter of a required area.